Relationship between endosomes and lysosomes.
In cell biology, an endosome is a membrane-bound compartment inside eukaryotic cells. It is a The LDL remains in the endosome and is delivered to lysosomes for processing. Early endosomes consist of a dynamic tubular- vesicular network (vesicles up to 1 µm in diameter with connected tubules of approx. 50 nm. lysosomes with both endosomes and the plasma membrane and on the sorting They are defined as membrane bound organelles of б/. mm diameter evidence for a dynamic relationship between the lumenal and limiting membrane . The intent in initiating this volume was to bring together a series of essays which would define our present understanding of the endosome and lysosome and.
In addition, autophagosomes execute autophagy, which delivers intracellular contents to the lysosome.
In the endocytic pathway, gradual maturation of endosomes into a lysosome and acidification of the late endosome are accompanied by vesicle trafficking, protein sorting and targeted degradation of some sorted cargo. Two opposing sorting systems are operating in these processes: The endosomal-lysosomal system is emerging as a central player in a host of neurodegenerative diseases, demonstrating potential roles which are likely to be revealed in pathogenesis and for viable therapeutic strategies.
Thus, the endosomal-lysosomal system is emerging as a key to understanding the mechanisms underlying both protein degradation and neurodegeneration. Here, we intend to summarize advances in the study of the endosomal-lysosomal system, with a focus on compartmentalized organization of trafficking routes, sorting machinery and their relationships to neurodegeneration.
Macromolecules and transmembrane proteins at the plasma membrane destined for degradation could enter the endosomal-lysosomal system via three broadly defined routes: Following fusion, lysosomes can be re-formed from endolysosomes EL by a maturation process involving content condensation and retrieval pathways removing endosomal membrane proteins and recycling SNAREs.
The fusion of late endosomes with lysosomes would consume both organelles if no recovery process occurred, and thus lysosome re-formation from endolysosomes is a necessary, although poorly understood process. Thus, the V-ATPase in the lysosomal membrane appears not only to be responsible for the well-described function of creating the acidic environment for macromolecule hydrolysis by lysosomal hydrolases, but also to generate dense-core lysosomes Hirota et al.
During the re-formation of lysosomes from endolysosomes, there is also membrane retrieval to remove endosomal membrane proteins and recycle SNAREs.
The endosomal-lysosomal system: from acidification and cargo sorting to neurodegeneration
Consistent with this, live-cell microscopy has shown vesicular tubular structures leaving endolysosomes after endosome—lysosome fusion Bright et al. In the re-formation of lysosomes from autolysosomes formed by autophagosome—lysosome fusion Jahreiss et al.
Some additional clues as to the machinery of lysosome re-formation have come from observations of cells from patients with lysosomal storage diseases. These are rare, inherited genetic defects, in many cases causing deficiencies in specific lysosomal acid hydrolases but in others resulting in defects in lysosomal membrane proteins or nonenzymatic soluble lysosomal proteins. The lack of efficient degradation of macromolecules may itself prevent lysosome re-formation Bright et al.
The mechanism by which Niemann—Pick type C2 may function to prevent lysosome re-formation is not known. Finally, the absence of the lysosomal cation transporter mucolipin-1 in cells from patients with mucolipidosis type IV has been suggested to result in a failure to re-form lysosomes from endolysosomes Treusch et al.
Nonactive TFEB is highly phosphorylated and is bound to the cytosolic surface of lysosomes, but under specific conditions, such as starvation or lysosome dysfunction, it becomes dephosphorylated and is rapidly translocated to the nucleus for review, see Settembre et al. The pathway by which most of the newly synthesized acid hydrolases are delivered to lysosomes in mammalian cells has been known for many years for review, see Kornfeld and Mellman ; Ghosh et al.
On the other hand, EGF and the EGF receptor have a pH-resistant bond that persists until it is delivered to lysosomes for their degradation. The mannose 6-phosphate receptor carries ligands from the Golgi destined for the lysosome by a similar mechanism.
Endosome - Wikipedia
Types[ edit ] Endosomes comprise three different compartments: Once endocytic vesicles have uncoated, they fuse with early endosomes. Early endosomes then mature into late endosomes before fusing with lysosomes. They become increasingly acidic mainly through the activity of the V-ATPase.
Loss of these tubules to recycling pathways means that late endosomes mostly lack tubules. They also increase in size due to the homotypic fusion of early endosomes into larger vesicles. Removal of recycling molecules such as transferrin receptors and mannose 6-phosphate receptors continues during this period, probably via budding of vesicles out of endosomes. Lysosomes reform by recondensation to their normal, higher density. However, before this happens, more late endosomes may fuse with the hybrid.
Some material recycles to the plasma membrane directly from early endosomes,  but most traffics via recycling endosomes.
Endosomes and Lysosomes: A Dynamic Relationship - Google Livres
Late endosomes, also known as MVBs, are mainly spherical, lack tubules, and contain many close-packed lumenal vesicles. Phagosomesmacropinosomes and autophagosomes  mature in a manner similar to endosomes, and may require fusion with normal endosomes for their maturation.
Some intracellular pathogens subvert this process, for example, by preventing RAB7 acquisition.